1. Field of the Invention
The present invention relates to fluorinated photopolymers having one or more photosensitizers incorporated into the polymer. Such photopolymers are particularly useful for patterning organic electronic and biological materials.
2. Discussion of Related Art
Organic electronic devices offer significant performance and price advantages relative to conventional inorganic-based devices. As such, there has been much commercial interest in the use of organic materials in electronic device fabrication. Specifically, organic materials such as conductive polymers can be used to manufacture devices that have reduced weight and drastically greater mechanical flexibility compared to conventional electronic devices based on metals and silicon. Further, devices based on organic materials are likely to be significantly less damaging to the environment than devices made with inorganic materials, since organic materials do not require toxic metals and can ideally be fabricated using relatively benign solvents and methods of manufacture. Thus, in light of these superior weight and mechanical properties, and particularly in light of the lowered environmental impact in fabrication and additionally in disposal, electronic devices based on organic materials are expected to be less expensive than devices based on conventional inorganic materials.
Fabrication of electronic devices, whether from organic or inorganic materials, requires the creation on an industrial scale of precisely defined patterns of the organic or inorganic active materials in these devices, often at a microscopic level. Most commonly, this is accomplished by “photolithography,” in which a light-sensitive “photoresist” film that has been deposited on a substrate is exposed to patterned light. Although this can be done in numerous ways, typically a microscopic pattern of light and shadow created by shining a light through a photographic mask is used to expose the photoresist film, thereby changing the chemical properties of the portions of the photoresist that have been exposed to light. In a “positive” photoresist, the portions of the photoresist that are exposed to light become soluble in the “developer” solution that is then applied to the exposed photoresist, and the light-exposed portions of the photoresist are washed away (“developed”) by the developer solvent to leave a pattern of unexposed photoresist and newly exposed underlying substrate. A “negative” photoresist is treated as for a positive photoresist; however, in a negative photoresist, it is the unexposed rather than the exposed portions of the photoresist that are washed away by the developing.
In a standard process, the photoresist material is laying on top of an active material layer that is to be patterned. Once the development has taken place, the underlying layer is etched using either a liquid etchant or a reactive ion plasma (RIE) with the appropriate etch chemistry. In either case, the photoresist layer blocks the etching of active material directly beneath it. Once the etching is complete, the resist is stripped away, leaving the pattern of active material on the substrate.
Alternatively, the photoresist can be used with a so-called “liftoff” technique. In this case, the resist is processed on a substrate before the active material layer is deposited. After the photoresist pattern is formed, the active material is deposited on both the substrate and the photoresist. In an additional “lift-off” or “stripping” step, remaining photoresist along with an overlying layer of active material is removed via the appropriate solvent to leave the desired patterned active material.
Although the use of photoresists is routine in traditional electronic devices based on inorganic materials, photolithography has been difficult to obtain for devices using organic materials, thereby hindering the development of devices based on these materials. Specifically, organic materials are much less resistant to the solvents that are used for conventional photolithography, as well as to the intense light sources that are used in these processes, with the result that conventional lithographic solvents and processes tend to degrade organic electronics. Although there have been various attempts to overcome these problems, e.g., by ink jet printing or shadow mask deposition, these alternative methods do not produce the same results as would be obtained with successful photolithography. Specifically, neither ink jet printing nor shadow mask deposition can achieve the fine pattern resolutions that can be obtained by conventional lithography, with ink-jet printing limited to resolutions of approximately 10-20 μm and shadow mask deposition to resolutions of about 25-30 μm.
US 2011/0159252 discloses a useful method for patterning organic electronic materials by an “orthogonal” process that uses fluorinated solvents and fluorinated photoresists. The fluorinated solvents have very low interaction with organic electronic materials.
Although the orthogonal process has made good progress, these fluorinated systems not always have sufficient sensitivity to the exposing radiation, especially in the range of 330 to 450 nm. Many conventional photoresist compositions include a photosensitizing additive, commonly referred to as a sensitizer or sensitizing dye, to increase the photosensitivity of the photoresist at a particular wavelength. By varying the amount of sensitizer added to the photoresist, the photo speed and spectral sensitivity of the system can be modulated. An important technical limitation of most existing sensitizers is that they are not highly soluble in fluorinated coating solvents or fluorinated developing solutions. Consequently, the concentration of sensitizer that can be employed in fluorinated photoresist composition is very limited and development can leave behind a residue of the sensitizer. Secondly, some sensitizers are susceptible to sublimation during the baking process, thereby depleting the photoresist formulation of sensitizer. In addition, the sublimed sensitizer can coat the baking tools and then flake off during the subsequent processing, resulting in further problems in the system.
In light of the above, there is a need to provide a more effective sensitization for use with fluorinated photoresists/fluorinated solvent systems.